In the fabrication of integrated circuits, polysilicon gates are deposited on silicon substrates and source/drain junctions implanted into the silicon substrates to form the transistors. Various interlayer dielectric materials are deposited on the gates and junctions for the formation of various layers of metal interconnections. Then various etching steps are used to form holes through the interlayer dielectric materials into which conductive materials will be deposited to form the contacts and interconnections of the integrated circuits.
The etching steps need to take place in a highly selective fashion with respect to the underlying layer, i.e., the etch for a layer should remove that layer relatively quickly while not removing or very slowly removing the underlying layer. To obtain good etch selectivity, fluorocarbon plasmas containing high carbon to fluorine (C/F) ratios are usually employed.
Unfortunately, polymerization readily occurs in chemistries having high C/F ratios.
This tends to generate quantities of polymers that are not easily removed. Even the ion bombardment is insufficient to remove the polymers. The mechanism for obtaining high etch selectivity is the difference in formation of polymers on different materials.
For example, with a silicon dioxide (SiO.sub.2) dielectric layer and a silicon nitride (Si.sub.3 N.sub.4) etch stop layer, a dual-frequency driven plasma source may be used for the contact etch process because it can achieve high etch selectivity of SiO.sub.2 -to-Si.sub.3 N.sub.4. The plasma contains a high C/F ratio which creates reactive unsaturated polymers which can stick easily to contact hole sidewall and bottom and, then, create thick polymer layers. Once the SiO.sub.2 dielectric layer is cleared over the Si.sub.3 N.sub.4 stop layer, and oxygen is no longer being brought into the local environment from the SiO.sub.2 etch process, the polymer tends to accumulate at a significant rate.
In SiO.sub.2 etch, using C.sub.x F.sub.y as etchant, free fluorine is made responsible for the etch, forming SiF in the reaction with silicon. CF and CF.sub.2 are precursors for the formation of the fluorocarbon polymers. This fluorocarbon layer deposition is reduced on oxide surfaces by the reaction between the oxygen released by dissociation during SiO.sub.2 etching and the carbon in the deposited polymer. Volatile compounds such as CO, CO.sub.2 or COF.sub.2 are formed during the etch. On non-oxide surfaces (or Si.sub.3 N.sub.4 surface) polymer layers should be formed, protecting the surface from free fluorine.
However, a potential drawback of leaving a thick polymer layer over Si.sub.3 N.sub.4 is to lower the etch selectivity of Si.sub.3 N.sub.4 -to-silicide (TiSi.sub.x). The silicide is formed on the polysilicon gate to provide low contact resistance bonding to the conductive contact metals. Fluorine from the deposited polymer can reduce, substantially, the etch selectivity of Si.sub.3 N.sub.4 -to-TiSi.sub.x on fully processed wafers. This can cause open or high contact resistances.
A method for increasing the etch selectivity and the process window for etching semiconductor contacts has been long sought, but has long eluded those skilled in the art.